This invention relates generally to the separation of gas mixtures by pressure swing adsorption and relates more particularly to the separation of air into its constituents by pressure swing adsorption.
Conventional pressure swing adsorption processes for generating oxygen from an airstream delivered to the system commonly include a fixed bed of adsorbent material adapted to adsorb nitrogen from air routed through the bed so that an oxygen-rich product gas exits the bed. The principles of separation involved in such an adsorption system are based upon equilibrium separation. However, due to the presence of argon in air, typically 1.0 percent by volume, which is not normally adsorbed by the bed material, the percentage by volume of argon in the product stream, assuming that all of the nitrogen is adsorbed by the bed material, is about 5.0 percent. Therefore, pressure swing adsorption processes which employ the principles of equilibrium separation utilizing air as the feed stream cannot normally generate a product stream containing an oxygen concentration which is appreciably greater than 95.0 percent. Oxygen of a purity greater than 95.0 percent is preferred, however, in welding and cutting processes which require oxygen and some medically-related applications. Accordingly, it would be desirable to provide a pressure swing adsorption process capable of generating a product stream containing an oxygen concentration which is greater than 95.0 percent from an air feed stream.
One pressure swing adsorption system is known which can produce a product stream containing an oxygen concentration of relatively high purity. Such a system, shown and described in U.S. Pat. No. 4,190,424, includes a pair of beds of molecular sieve carbon and a pair of beds of zeolite molecular sieve. The beds are joined in flow communication by a relatively complicated network of conduits, valves and reservoirs, and a multiplicity of compressors. During operation of the system, a feed stream of air constituents (i.e., oxygen, nitrogen and argon) is delivered to the carbon beds where oxygen is adsorbed at a higher rate than are nitrogen and argon. The carbon beds subsequently undergo a purge step so that a purge stream which is rich in oxygen and depleted in nitrogen and argon flows out of the carbon beds. At least a portion of this purge stream is subsequently routed as feed stock through the zeolite beds where nitrogen is further adsorbed so that the product stream which exits the zeolite beds is high in oxygen purity.
A limitation associated with the system of the referenced patent relates to the relatively complicated network of conduits, valves and compressors utilized to move selected gas mixtures between and through the beds. In fact, the system includes no less than twenty valves and three pumps in order to operate in its desired manner. Due to the amount of valve componentry involved and the power requirements of the three pumps when the system is in use, the system is costly to construct and operate. It would be desirable to provide a system of the aforedescribed type which utilizes a less complicated network of conduits and valves and a fewer number of pumps, or compressors.
Another limitation associated with the system of the referenced patent relates to the means provided for removing moisture or like contaminants from the feed stream of air constituents delivered to the system. More specifically, such means call for the venting of part of the gas evacuated from the beds of the zeolite molecular sieve. Under normal operating conditions, the gas evacuated from the zeolite sieve beds contains not only the moisture but a relatively high percentage (approximately 80 percent by volume) of oxygen as well. Therefore, by venting gas evacuated from the zeolite sieve beds, the system experiences a substantial loss of oxygen. Consequently, the overall efficiency of the system to generate oxygen suffers from the venting of this stream. It would be desirable to provide a system wherein moisture contained within the feed stream of air delivered to the system is expelled therefrom with no appreciable sacrifice in the overall efficiency of the system to generate oxygen.
Accordingly, it is an object of the present invention to provide a new and improved pressure swing adsorption process and system for separating a preselected constituent from a gas mixture containing air constituents.
Another object of the present invention is to provide such a process and system which combines the principals of kinetic and equilibrium separation for the purpose of separating air into separate quantities of high purity oxygen and nitrogen.
Still another object of the present invention is to provide such a process and system for producing a product stream containing an oxygen concentration of at least 99.5 percent by volume.
A further object of the present invention is to provide such a system with an effective means of moisture removal without sacrificing the efficiency of the system to generate oxygen.
A still further object of the present invention is to provide such a system which is relatively compact and uncomplicated in construction and economical to construct and operate.